I've actually turned a servo is a very primitive speed controller quite some time ago. I took out the motors and gears, and connected the motor leads to the trigger leads of a relay (I think it was a 9v relay from my 140-in-1 ket) with the battery/motor to the current side. It was then possible to adjust the trim so that it would be sitting still and you could hear the relay clicking open and shut very quickly, making it sound like there was something revving in the car. When I pulled the throttle, the relay would engage sending the power from the battery to the motor - the only problem being it was always full power, there was no throttle control whatsoever.. It was fun for a while to tear around with it in a buggy...